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  \item{x}{a formal argument.}
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  \item{x}{a formal argument.}
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}
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}
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\details{
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\details{
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  \code{missing(x)} is only reliable if \code{x} has not been altered
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  \code{missing(x)} is only reliable if \code{x} has not been altered
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  since entering the function: in particular it will \emph{always}
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  since entering the function: in particular it will \emph{always}
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  be false after \code{x <- match.arg(x)}.
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  be false after \code{x <- match.arg(x)}.
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  The example shows how a plotting function can be written to work with
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  The example shows how a plotting function can be written to work with
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  either a pair of vectors giving x and y coordinates of points to be
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  either a pair of vectors giving x and y coordinates of points to be
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  plotted or a single vector giving y values to be plotted against their
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  plotted or a single vector giving y values to be plotted against their
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  indices.
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  indices.
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  Currently \code{missing} can only be used in the immediate body of
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  Currently \code{missing} can only be used in the immediate body of
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  the function that defines the argument, not in the body of a nested
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  the function that defines the argument, not in the body of a nested
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  function or a \code{local} call.  This may change in the future.
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  function or a \code{local} call.  This may change in the future.
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  This is a \sQuote{special} \link{primitive} function: it must not
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  This is a \sQuote{special} \link{primitive} function: it must not
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  evaluate its argument.
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  evaluate its argument.
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}
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}
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\references{
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\references{
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  Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
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  Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)